1. Field of the Invention
The present invention relates to the technical field of audio processing and, more particularly, to an automotive virtual surround audio system.
2. Description of Related Art
Current playback contents such as a CD, an MP3 and a broadcast are rendered with left- and right-channel audio sources. However, an automotive audio system is equipped with four speakers, i.e., front-left (FL), rear-left (RL), front-right (FR) and rear-right (RR). Accordingly, the prior art typically sends the left-channel audio source to the front-left (FL) and rear-left (RL) speakers and also the right-channel audio source to the front-right (FR) and rear-right (RR) speakers. Such a way lacks of the surround effect because the tandem playback content is very similar, without completely applying the benefit of multi-channel audio.
The speakers of the automotive audio system typically are positioned in the car doors. In addition, due to a listener does not seat at a center position of the car, the perceptual audio to the listener is tilted to a certain side and has a lower elevation. The perceptual audio for the listener at the rear seat is directed typically by the speaker behind the head. Namely, the music is from the rear in listening. When viewing a multi-channel movie, only the surround audio is played, and the front audio sources responsible for positioning cannot be reproduced clearly, which causes the positioning uncertainty.
Therefore, U.S. Pat. No. 6,501,843 granted to Usui, et al. for an “Automotive audio reproducing apparatus” has disclosed a head related transfer function (HRTF) to find corresponding inverse filters. However, such a technique assumes that a plant is symmetric and locates a dummy on the center of an automobile for measurement to thereby obtain the parameters associated with the plant. In this case, only two channel input signals and the front listeners are considered. In addition, a rear speaker plays a low frequency signal only, which does not sufficiently apply the features of a multi-channel speaker system.
U.S. Pat. No. 7,206,413 granted to Eid, et al. for a “Sound processing system using spatial imaging techniques” has disclosed a crossbar matrix mixer to convert N input signals into M output signals. However, such a technique only uses different weights to mix the channel signals, which cannot overcome the problem that the listener is affected essentially by the audio of the closest speaker. In addition, the positioning required by the multi-channel content (such as DVD) is uncertain due to the mixed audio.
U.S. Pat. No. 7,164,773 granted to Fabry for a “Vehicle electroacoustical transducing” has disclosed seven electroacoustical transducers, including four being positioned at four doors, one at the front windshield and two at the rear parcel shelf behind rear seats in an automobile. Three transducers at the front seats provide the front-left (FL), the front-right (FR) and the front center channel audio sources respectively, two transducers on the rear parcel shelf provide the rear-left and the rear-right channel audio sources respectively, and two transducers at the rear doors provide the surround channel audio sources for the front passengers and the front channel audio sources for the rear passengers, which can play all audio signals. For example, the front-left, the rear-left and the front center channel audios can be rendered at the left side. Such a way essentially positions a certain number of transducers or speakers in the compartment of the automobile to thereby obtain the multi-channel audio system. However, as cited above, the number of used speakers is higher. In addition, the asymmetry between a listener and the speakers still exists, and the problem of position confusing can easily occur when the two speakers at the rear doors provide a mixed audio containing all signals.
U.S. Pat. No. 5,193,118 granted to Latham-Brown, et al. for a “Sound processing system using spatial imaging techniques” has disclosed four gamut speakers placed at four doors, and a woofer placed below a front seat. The two-channel input signal is outputted to the speakers directly. For example, the left channel signal is sent to the front-left and the rear-left speakers. Such a way only defines the location of the speakers and cannot overcome the problem of poor listening compartment in an automobile. In addition, this patent only focuses on two channel inputs, and the channel extension technique therein copies the original two channels to the ambient channel only.
Therefore, it is desirable to provide an improved system to mitigate and/or obviate the aforementioned problems.